In electrostatic printing a latent image is formed by scanning a laser or light from an LED (light emitting diode) on a continuous receiving surface, such as a photoconductive belt or drum. The receiving surface is first charged with a uniform negative or positive charge, and where the light strikes the surface the charge is conducted away, leaving the latent image. The image is then developed by the electrical attraction of toner particles to the latent image areas on the photoconductive surface.
In electrostatic imaging devices the toner particles are often contained in and supplied through replaceable cartridges. For color printing, several colors of toner, such as cyan, magenta, yellow and black, are utilized to create the desired color gamut. Each color is typically contained in a separate cartridge. Mono-component contact development refers to the use of a single type of toner particle, as opposed to dual-component development which uses carrier particles and toner particles that are mixed together prior to contact with the belt.
In mono-component contact development, the size of the individual toner particles in a given cartridge typically varies. As the smaller particles have a lower mass and a higher charge per mass, they are more easily attracted to the photoconductive surface and are generally developed first at the beginning of the life of the cartridge. As the smaller particles are depleted over time, the larger toner particles are used more toward the end of the useful life of the cartridge. This causes a density shift in the developed image over the life of the toner cartridge, as the large particles tend to create a darker image. Also, as the usage of a toner cartridge increases, the charge control agent in the toner is depleted and the particles attain a lower electrical charge. This results in a greater number of toner particles being developed for a given latent image, thereby further darkening the developed image. In some cases the optical density of a printed image can increase 20% to 30% from the beginning of a new developer cartridge to the end of its useful life. In color printing, this density shift can cause a major change in the color tone of a composite image, such as when one of four color toner cartridges is replaced with a new cartridge.
In the prior art, separate toner development sensors in the printing apparatus have been utilized in an attempt to control image density. These components add unwanted cost and complexity to the apparatus and generally give poor results. The present invention seeks to overcome the shortcomings of the prior art by providing a reliable method for controlling image density that eliminates the need for additional sensory and logic components.